[GHC] #8913: either bug or confusing error message mixing PolyKinds and TypeFamilies

[GHC]

#8913: either bug or confusing error message mixing PolyKinds and TypeFamilies
-------------------------------------+-------------------------------------
        Reporter:  ghorn             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler (Type    |          Version:  7.8.1-rc2
  checker)                           |         Keywords:  PolyKinds,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+-------------------------------------

Comment (by goldfire):

 With `-fprint-explicit-kinds` we see that this really looks like a proper
 inference bug:

 {{{
     Could not deduce (GCat * (MyRep a)) arising from a use of ‘gcat’
     from the context (GCat * (MyRep a), MyGeneric a)
       bound by the type signature for
                  cat :: (GCat * (MyRep a), MyGeneric a) => a -> Int
       at Scratch.hs:12:8-48
     In the expression: gcat (from x)
     In an equation for ‘cat’: cat x = gcat (from x)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8913#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler